Conventional neo-Darwinian theory views organisms as infinitely sensitive and responsive to their environments, and considers them able to readily change size or shape when they adapt to selective pressures. Yet since 1863 it has been well known that Pleistocene animals and plants do not show much morphological change or speciation in response to the glacial–interglacial climate cycles. We tested this hypothesis with all of the common birds (condors, golden and bald eagles, turkeys, caracaras) and mammals (dire wolves, saber-toothed cats, giant lions, horses, camels, bison, and ground sloths) from Rancho La Brea tar pits in Los Angeles, California, which preserves large samples of many bones from many well-dated pits spanning the 35,000 years of the Last Glacial–Interglacial cycle. Pollen evidence showed the climate changed from chaparral/oaks 35,000 years ago to snowy piñon-juniper forests at the peak glacial 20,000 years ago, then back to the modern chaparral since the glacial–interglacial transition. Based on Bergmann's rule, we would expect peak glacial specimens to have larger body sizes, and based on Allen's rule, peak glacial samples should have shorter and more robust limbs. Yet statistical analysis (ANOVA for parametric samples; Kruskal–Wallis test for non-parametric samples) showed that none of the Pleistocene pit samples is statistically distinct from the rest, indicating complete stasis from 35 ka to 9 ka. The sole exception was the Pit 13 sample of dire wolves (16 ka), which was significantly smaller than the rest, but this did not occur in response to climate change. We also performed a time series analysis of the pit samples. None showed directional change; all were either static or showed a random walk. Thus, the data show that birds and mammals at Rancho La Brea show complete stasis and were unresponsive to the major climate change that occurred at 20 ka, consistent with other studies of Pleistocene animals and plants. Most explanations for such stasis (stabilizing selection, canalization) fail in this setting where climate is changing. One possible explanation is that most large birds and mammals are very broadly adapted and relatively insensitive to changes in their environments, although even the small mammals of the Pleistocene show stasis during climate change, too.

This issue was thoroughly discussed by Stephen Jay Gould in his 2002 book "The Structure of Evolutionary Theory." He began by quoting a paper by Wake, Roth and Wake (1983) where they asked ...

"With natural selection operating in a changing environment as an agent of adaptation, we expect to see changes at the organismal, ultimately physiological and morphological, level. How, though, can we explain the paradoxical situation in which environments change, even dramatically, but organisms do not?"

Stabilizing selection is not an explanation for stasis as Gould notes on page 878.

... although we all acknowledge stabilizing selection as too important and pervasive a phenomenon to hold no relevance for this issue, a complete explanation of stasis in these conventional terms seems implausible both on empirical grounds, and also by the basic logic of proper scaling.

As often emphasized in this chapter, if stasis merely reflects excellent adaptation to environments, then why do we frequently observe such profound stasis during major climatic shifts like ice-age cycles (Cronin, 1985), or through the largest environmental change in a major interval of time (Prothero and Heaton, 1996)?

It looks like stasis in the face of environmental change has been a well-established fact for many decades. This should not be a surprise to anyone who studies evolution [The Paradox of Stasis?].

The premise—that evolution=adaptation and it is driven by environmental change—is obviously wrong as a generalization.

[Photo credit: I took this picture in February 2012 when I visited the tar pits; La Brea Tar Pits.]

34 comments
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Not a single extinction mentioned. Many relevant ecological conditions (predators, prey, competitors) seemed to have remained amazingly constant. How severe was the climate change in coastal California? And how much change would be expected from current measures of latitudinal size differences (Bergmann's Rule). Were samples corrected for sex and age classes? The samples seem very variable, and it would be interesting to know what sample size would be necessary, for example, to detect a 5% difference. It is good to remember that good old Homo sapiens, despite a few thousand years in a very new cultural climate, also seems to not have evolved much morphologically, although our genes show that we have.

It seems to me that, absent other evidence not presented here, that the most that can be said is that morphological changes among specimens recovered seems minimal. That should in no way be equated (ala Adnan Oktar, aka Harun Yahya) that evolution mysteriously ceased in those instances-- whether for the Pleistocene pit samples that are the primary data point here (26 My period), or over time periods longer by an order of magnitude or more (think horseshoe crabs). Absent DNA sequence data obtained from the samples, any judgment regarding the pace of evolutionary change would seem hopelessly unsubstantiated.

This yEC is not surprised because the tar pits only represent the 1700's BC or so.then diversity was greater in the past as biological systems were superior. The bible implies this.So our present creatures are just the survivors of those more diverse times or simply smaller versions.I know they say buffalos are just smaller versions of their ancestors back then and not a surviving type.

Yet by evolutionary standards there should be more diversity up and down in these creatures. One can't have glorious evolution of bugs to buffalos and then say stasis is a option too.Otherwise the hypothesis just is flexible for anything and explains nothing.The researchers were right about being surprised.It should not be so fixed as to imply there is no evolution going on or on its way with these critters.Evolutionary biology is not true.

When he died in 2002, Gould claimed that he no longer believed that "constraint" was the explanation for stasis (TSET p. 880).

His preferred model at that time was that species with huge population sizes—that's mostly what they're looking at—are subdivided into many smaller demes. These demes might adapt slightly to local environments but the adaptations can't spread to other demes in slightly different environments. The net effect is that the species as a whole doesn't change very much even though there might be small localized changes in different parts of the range.

You'll have to read Gould and others for a more detailed view of current thinking about stasis. The important point is that most species are doing quite fine in their current environment and they have enough buffering capacity to withstand fairly substantive changes in the environment. If the change is too drastic they move (habitat tracking).

It's wrong to think that most species are so fine-tuned to their existing environment that any small environmental change requires adaptations in order to avoid extinction. On the other hand, it's clear that some catastrophic changes will cause mass extinctions.

Seems as if experiments in microorganisms could be relevant here. Some favorable mutations in microorganisms take place over such short time frames that some experimenters thought it appeared to be directed. But long term experiments by Lenski, for example, show bacteria taking tens of thousands of generations to hit the "right" combination of mutations to adapt well to certain environments. Seems as if even in microorganisms some mutations and combinations thereof are much more easily achieved and propagated through a population than others. So perhaps this might be one factor, along with lack of sufficient breeding isolation, etc.

LM writes The important point is that most species are doing quite fine in their current environment and they have enough buffering capacity to withstand fairly substantive changes in the environment. If the change is too drastic they move (habitat tracking).

Somewhat confused. Do plants also have some form of habitat tracking? I'm trying to work out how the above statement is consistent with the vast variety and array of plant life.. It seems like plants would just go extinct, rather than adapt as prolifically as they have. Hundreds, if not thousands, of plants all inhabit the same plots of land, presumably all pretty much needing the same type of nutrients (water, sunshine, particular soil minerals), to survive. If they are all 'doing quite fine', why did they differentiate so much?

NE, thanks, I see that. But then, would they also move back again when the climate change restored itself? I'm still trying to figure out why so many species cohabit the same plot of land. It seems like the ones that had remained would be able to repel plants that tried to return.

Still trying to make this fit with 'doing quite fine in their current environment' which seems to indicate diversification of life form as an exception rather than a rule.

Andy wrote:"But then, would they also move back again when the climate change restored itself?"

Presumably, yes. In fact, there are many indications that the Amazon forest is still undergoing ecological succession from a few loci that are richer in biodiversity and that map well to what is thought was the spots that managed to "survive" the last Ice Age. The same can be said about much of the taiga in the northern latitudes, which also seem to be moving "north" and undergoing ecological succession since the last Ice Age.

Andy wrote:"It seems like the ones that had remained would be able to repel plants that tried to return"

That, of course, is possible, but complicated to gauge in general terms. You have to take into account all the complexities of competition theory and look in practice at specific events to see what's happening. But there will certainly be some competition.

Andy wrote:"Still trying to make this fit with 'doing quite fine in their current environment' which seems to suggest diversification of life form as an exception rather than a rule"

That depends. You have to consider that both the separation of small populations from a bigger one and genetic drift will allow nicelly for diversification to happen without any conflict with "doing quite fine in their current environment". The apparent conflict seems more propense (is this a word?) to happen if you are thinking in adaptationist terms.

Pedro, thanks.I definitely see your last point i.e., "The apparent conflict seems more propense (is this a word?) to happen if you are thinking in adaptationist terms.", in which case it really does seem, to me at least, to actually BE a contradiction. In terms of other causes for species diversification, particularly genetic drift, I confess I am still trying to work out in my head exactly how it works. So, I'm still puzzling over it, but your answer does help me quite a bit.

Yes, of course. In fact the habitat tracking of plants is the paradigm for the entire phenomenon.

We are seeing it right now as the treeline moves north in Canada and all kinds of plants are spreading into the valleys left by former glaciers. There's an entire subsection of ecology devoted to Ecological Succession.

Larry, thank you. Fascinating. Ecological Succession reminds me of an ongoing experiment taking place in an area of Colombia called Gaviotas. They are reforesting a deforested area, gradually restoring it to rainforest like conditions, by beginning with the groundlaying plants. The technique has turned out to be working more quickly there than even the scientists suspected it would. I think it's probably a fifty or sixty year old project at this point and various flora and fauna have already made their way back from the surviving rainforests in the region.

"Conventional neo-Darwinian theory views organisms as infinitely sensitive and responsive to their environments"

No it doesn't. There is no expectation that any species should be "infinitely sensitive" to changing selective pressures. That's why countless species go extinct. The rest of their "paper" is thereby destroyed.

The impacts section of the IPCC says "Protecting species that currently are vulnerable, endangered, or critically endangered (see Table 5-5) requires measures that, in general, reverse the trend toward rarity. Without management, there is high confidence that rapid climate change, in conjunction with other pressures, probably will cause many species that currently are classified as critically endangered to become extinct and several of those that are labeled endangered or vulnerable to become much rarer, and thereby closer to extinction, in the 21st century (Rabinowitz, 1981)."

Going extinct in the face of a slight change in the environment strikes me as being quite sensitive and responsive.Yeah but that's not what I meant, nor what I think was implied in their "paper". I took it to mean that Axe and co. thinks that evolution implies species will quickly adapt to even the most minute changes and so adaptation is infinitely minute, gradual, inevitable and smoothly follows the environment wherever it goes. It's essentially a strawman they erect, in order to point out how evolution must thereby be falsified when that doesn't happen.

Don't most biologists believe we're facing a crisis of extinction just because the average temperature of our planet will increase by a few degrees?Yeah, because species are not infinitely sensitive in their adaptive ability, especially over so short timescales.

When there's no more ice on the north-pole, noone but the ID strawman being erected is expecting polar-bears to become whales in 50 years or less. When polar bears aren't whales in 50 years, but extinct instead because they couldn't find land to stand on or food to eat(either because it migrated south or went extinct itself), the Discovery Institute will tout this as a falsification of evolution.

Incidentially, a minor adjustment in temperature is not a complete or very adequate description of an environment to make predictions from. If the average temperature changes from -1 to +1, well, large landmasses can disappear under water. And the amount of CO2 in the atmosphere also has consequences for the acidity of the oceans etc.

I should add that the paper wasn't written by the ID guys of course, only that they chose to highlight this paper because of the implication they see.

I still think it's weird to write that species are assumed to be "infinitely sensitive and responsive" to their environments. I'm not an evolutionary biologist, I'm a layman nobody, but I've read around quite a lot and I've never got the impression that this was a working assumption for anyone. It's definitely overstated.

I totally agree with Rumraket that it is weird to assume that populations are "infinitely sensitive and responsive" (and I am an evol. biol., fwiw). I don't think many evolutionary biologists think that adaptation is that simple.

I'm not, but then again I have a background in Geology and I was bombarded with the notion of Punctuated Equilibrium during my studies. I admit I always thought Stabilizing Selection was the answer to Stasis. Guess I need to catch up.

@Helen above:"Did Gould provide an explanation for stasis, other than unspecified constraints?"

I can't remember, as I refered above I thought it was stabilzing selection. I'd like to know that too. Anyone?

Stabilizing selection implies fine-tuning of a species to it's environment such that any deviation from the status quo will be eliminated because it reduces fitness. It suggests that all the observable phenotypic characteristics of a species are the best possible adaptations to the current habitat. In other words, most species sit atop an adaptive peak.

This is inconsistent with punctuated equilibria because the observation is that evolutionary change is coupled to cladogenesis where a new daughter species is formed but the parental species continues to exist in the same environment.

This gives rise to species sorting, according to Gould and others, because these species compete in the same environment. Cladogenesis wouldn't be possible if the changes in the new species were maladaptive as implied by stabilizing selection.

It's much more likely that most species are only part way up an adaptive peak and for many phenotpyic characteristics there may not even be an adaptive peak. The reason you don't see more phenotypic adaptation is because it doesn't confer very much fitness advantage in a species that's doing well enough. Stasis then becomes simply a consequence of huge population sizes where change takes a very long time.

"Conventional neo-Darwinian theory views organisms as infinitely sensitive and responsive to their environments"

Wrong. In the Modern Synthesis models, response to selection depends primarily on genetic variability, which is not "infinite" or necessarily high. And when molecular studies showed lots of variability in populations, that was not what many "neo-darwinians" expected.

"the data show that birds and mammals at Rancho La Brea show complete stasis and were unresponsive to the major climate change"

Only in the skeleton morphology. Who knows how those animals could have been evolving at their life history, metabolism, fur coats, behavior...

I am not at all surprised by the results of this paper. Even some evolutionary ecologists (e. g. Douglas Futuyma and Michael L. Rosenzweig) recognize the existence of long-term ecological stasis as well as evolutionary stasis, which this paper is an excellent demosntration of both.

I've got to say that I'm a little surprised, as I figured there would have been at least some variation among certain traits over that time. Then again, I'm not a biologist, so my surprise doesn't really count for much.

I think a large part of the concern about rising global temperatures is movement of organisms away from their usual habitat. They will become exposed to microorganisms which their immune systems are not familiar, and bring new microorganisms with which the endemic species are not familiar.

Laurence A. Moran

Larry Moran is a Professor in the Department of Biochemistry at the University of Toronto. You can contact him by looking up his email address on the University of Toronto website.

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Quotations

The old argument of design in nature, as given by Paley, which formerly seemed to me to be so conclusive, fails, now that the law of natural selection has been discovered. We can no longer argue that, for instance, the beautiful hinge of a bivalve shell must have been made by an intelligent being, like the hinge of a door by man. There seems to be no more design in the variability of organic beings and in the action of natural selection, than in the course which the wind blows.Charles Darwin (c1880)Although I am fully convinced of the truth of the views given in this volume, I by no means expect to convince experienced naturalists whose minds are stocked with a multitude of facts all viewed, during a long course of years, from a point of view directly opposite to mine. It is so easy to hide our ignorance under such expressions as "plan of creation," "unity of design," etc., and to think that we give an explanation when we only restate a fact. Any one whose disposition leads him to attach more weight to unexplained difficulties than to the explanation of a certain number of facts will certainly reject the theory.

Charles Darwin (1859)Science reveals where religion conceals. Where religion purports to explain, it actually resorts to tautology. To assert that "God did it" is no more than an admission of ignorance dressed deceitfully as an explanation...

Quotations

The world is not inhabited exclusively by fools, and when a subject arouses intense interest, as this one has, something other than semantics is usually at stake.
Stephen Jay Gould (1982)
I have championed contingency, and will continue to do so, because its large realm and legitimate claims have been so poorly attended by evolutionary scientists who cannot discern the beat of this different drummer while their brains and ears remain tuned to only the sounds of general theory.
Stephen Jay Gould (2002) p.1339
The essence of Darwinism lies in its claim that natural selection creates the fit. Variation is ubiquitous and random in direction. It supplies raw material only. Natural selection directs the course of evolutionary change.
Stephen Jay Gould (1977)
Rudyard Kipling asked how the leopard got its spots, the rhino its wrinkled skin. He called his answers "just-so stories." When evolutionists try to explain form and behavior, they also tell just-so stories—and the agent is natural selection. Virtuosity in invention replaces testability as the criterion for acceptance.
Stephen Jay Gould (1980)
Since 'change of gene frequencies in populations' is the 'official' definition of evolution, randomness has transgressed Darwin's border and asserted itself as an agent of evolutionary change.
Stephen Jay Gould (1983) p.335
The first commandment for all versions of NOMA might be summarized by stating: "Thou shalt not mix the magisteria by claiming that God directly ordains important events in the history of nature by special interference knowable only through revelation and not accessible to science." In common parlance, we refer to such special interference as "miracle"—operationally defined as a unique and temporary suspension of natural law to reorder the facts of nature by divine fiat.
Stephen Jay Gould (1999) p.84

Quotations

My own view is that conclusions about the evolution of human behavior should be based on research at least as rigorous as that used in studying nonhuman animals. And if you read the animal behavior journals, you'll see that this requirement sets the bar pretty high, so that many assertions about evolutionary psychology sink without a trace.

Jerry Coyne
Why Evolution Is TrueI once made the remark that two things disappeared in 1990: one was communism, the other was biochemistry and that only one of them should be allowed to come back.

Sydney Brenner
TIBS Dec. 2000
It is naïve to think that if a species' environment changes the species must adapt or else become extinct.... Just as a changed environment need not set in motion selection for new adaptations, new adaptations may evolve in an unchanging environment if new mutations arise that are superior to any pre-existing variations

Douglas Futuyma
One of the most frightening things in the Western world, and in this country in particular, is the number of people who believe in things that are scientifically false. If someone tells me that the earth is less than 10,000 years old, in my opinion he should see a psychiatrist.

Francis Crick
There will be no difficulty in computers being adapted to biology. There will be luddites. But they will be buried.

Sydney Brenner
An atheist before Darwin could have said, following Hume: 'I have no explanation for complex biological design. All I know is that God isn't a good explanation, so we must wait and hope that somebody comes up with a better one.' I can't help feeling that such a position, though logically sound, would have left one feeling pretty unsatisfied, and that although atheism might have been logically tenable before Darwin, Darwin made it possible to be an intellectually fulfilled atheist

Richard Dawkins
Another curious aspect of the theory of evolution is that everybody thinks he understand it. I mean philosophers, social scientists, and so on. While in fact very few people understand it, actually as it stands, even as it stood when Darwin expressed it, and even less as we now may be able to understand it in biology.

Jacques Monod
The false view of evolution as a process of global optimizing has been applied literally by engineers who, taken in by a mistaken metaphor, have attempted to find globally optimal solutions to design problems by writing programs that model evolution by natural selection.